The antecedents of type II diabetes, while still controversial, are thought to involve decreased insulin sensitivity and compensatory hypersecretion of insulin. Mexican Americans have a three-fold excess risk of type II diabetes and non-diabetic Mexican Americans are characterized by hyperinsulinaemia and insulin resistance. Few data exist, however, on whether there are defects in insulin secretion and/or clearance in this population. We examined insulin sensitivity, secretion and clearance using combined insulin and C-peptide measurements analysed by the minimal model technique of Bergman and colleagues in 10 non-obese, normoglycaemic Mexican Americans and 11 age, sex and obesity-matched non-Hispanic whites. Mexican Americans had significantly decreased insulin sensitivity (SI 4.06 s. 7.56, P = 0.017), higher first phase insulin secretion (1.03 nM vs. 0.72 nM) and decreased insulin clearance (0.099 vs. 0.161) than non-Hispanic whites. Thus, normal Mexican Americans have higher rather than lower insulin secretion suggesting that lower insulin sensitivity may be an early defect in this ethnic group. In addition, they have reduced insulin clearance. Moreover, insulin sensitivity and insulin clearance were positively correlated. We thus speculate that decreased insulin clearance may represent a further autoregulatory mechanism in addition to increased insulin secretion to compensate for decreased insulin sensitivity.